Medical imaging by film/screen systems is widely recognized as one of the most useful tools in medical diagnostics. Early X-ray images obtained with intensifying screens required considerable patient dose and provided relatively poor image quality. In modern practice, patient dose has decreased by several orders of magnitude and image quality, in terms of the visibility of potentially useful diagnostic information content, has increased significantly. Still, the continued demand for higher image quality and lower dose is evidenced by continuing efforts to improve both films and intensifying screens.
It is well known in the art that the penetrating power of X-rays is roughly proportional to the beam energy. The fraction of the X-ray energy absorbed by a material decreases as the energy of the X-ray beam increases. As the penetrating power of the X-ray beam is increased, the difference in X-ray absorption between different materials in the exposed subject matter decreases and thus the X-ray contrast decreases. High energy exposures, e.g., 120 kVp, can be utilized where larger differences in X-ray absorption are present in the subject matter. This typically occurs when materials of different densities are present in the subject. One example is thoracic radiology, where the ribs and the mediastinum have high X-ray absorption and the lungs have a low X-ray absorption. In instances where the subject matter does not have large differences in X-ray absorption, low energy X-rays are used to enhance X-ray contrast. Low energy X-rays are typically used for soft tissue radiographs, such as mammography.
To increase the speed of the film/screen system, radiographs are typically obtained with a system comprising two screens with a film between them. The film comprises photographic emulsions coated on both sides of a support and is typically referred to in the art as "double emulsion." A double emulsion system has been difficult to employ for low energy X-ray systems. The screen closest to the X-ray source absorbs a large portion of the X-radiation and there is not a sufficient amount of radiation reaching the furthest screen to generate an acceptable image.
Because low energy X-rays are strongly absorbed by the X-ray phosphors, soft tissue radiography, i.e. mammography, generally utilizes single screens in combination with films coated on one side of a support. Thus, the speed of the film/screen systems utilized in soft tissue radiographs is generally lower than that of systems used in general radiography, leading to higher patient dose for mammography exams.
There has been an ongoing effort in the art to reduce patient dose and circumvent the problem associated with two-screen mammography systems. Efforts to avoid strong X-ray absorption of a front screen have been described in U.S. Pat. No. 4,710,637. This patent teaches the use of a high density phosphor with a high cross-section of X-ray absorption and high X-ray-to-light conversion efficiency of the front screen. The thickness of the front screen, as well as the coating weight of the phosphor it contains, is held very low so that a sufficient fraction of the incident X-ray energy is available to the back screen. In this configuration, high image sharpness, as measured by the Modulation Transfer Function (MTF), is achieved. Furthermore, this patent teaches that a back screen with the same type phosphor as the front is preferred. Lower MTFs are permissible for the back screen, as are greater intensifying screen thickness or coating weights. This permits phosphors of somewhat lower X-ray absorption efficiency and/or X-ray-to-light conversion efficiency to be employed in the back screen.
As is known in the art, the coating weights of the two screens can be advantageously adjusted so that similar fractions of the incident X-ray energy are absorbed by each of the screens. For example, if the front screen coating weight is adjusted to absorb 40% of the energy of an incident 28 kVp X-ray beam produced by a molybdenum anode, then the back screen coating weight is adjusted to absorb 40/(100-40), or 67%, of the remaining incident X-ray energy.
The solution suggested by U.S. Pat. No. 4,710,637 has a significant disadvantage because the phosphor coating weight in the front screen must be extremely low. This low coating weight can only be accomplished with a very thin front screen phosphor coating. The thin coating leads to coating non-uniformities in thickness and in phosphor distribution in the screen and drying patterns which lead to a high degree of image mottle, generally called "structure noise." Furthermore, the reproducible manufacture of very thin front screens is very difficult to maintain. Therefore, because of coating non-uniformities in the thin front screen, and other reasons, mammography systems utilizing the thin front/thick back screen have been of minor consequence in practical applications.